1. Field of the Invention
The invention relates to a liquid crystal material and applications thereof, and more particularly to a reflective liquid crystal material formulation and single color, multi-color, or full color reflective bistable liquid crystal displays employing the same.
2. Description of the Related Art
Liquid crystal display (LCD) can include various liquid crystal materials such as twisted nematic (TN) liquid crystals, super twisted nematic (STN) liquid crystals, ferroelectric liquid crystals (FLC), or cholesteric liquid crystals.
Cholesteric liquid crystals, otherwise called chiral nematic liquid crystals, are generally prepared by mixing a twisted nematic liquid crystal with a chiral agent (chiral dopant). Due to the chiral agent, cholesteric liquid crystals exhibit higher twisting angular than that of twisted nematic liquid crystals and super twisted nematic liquid crystals. Cholesteric liquid crystals have several states, including a homeotropic state in which the direction of the liquid crystal molecules is approximately parallel to the substrate, and two stable states of a planar state and a focal conic state with no voltage applied. Cholesteric liquid crystals exhibit a bistable state characteristic, and the two stable states are stable in the absence of an electric field. In the planar state and the focal conic state, the image data can be kept displayed as a still image with no voltage applied, except for the switching of the planar state and the focal conic state.
In the other hand, since cholesteric liquid crystals can reflect the surrounding light and can be used in the absence of backlight source and polarizer, the liquid crystal display employing the same has lower power consumption than normal liquid crystal display. Since the reflectance of liquid crystal composition can be modified depending on desired reflective wavelengths, the requirement of a color display can be achieved in the absence of color filter. The detail driving means and optical characteristics of cholesteric liquid crystals are disclosed in U.S. Pat. No. 5,251,048 and U.S. Pat. No. 5,695,682.
The cholesteric liquid crystal is composed of optically active chiral agent and nematic liquid crystal, with helical pitch controlled by the ratio therebetween. The relationship between helical pitch P and amount of optically active chiral agent is:P=1/(HTP·C)
In the above relational formula, C represents the weight concentration of chiral agent added to the liquid crystal composition and HTP helical twisting power, that is, the twisting and rotating ability of liquid crystal molecules. Therefore, the cholesteric liquid crystal with modifiable reflective wavelength can be obtained by adjusting the concentration of the chiral agent. The cholesteric liquid crystal composition with a higher concentration chiral agent has a lower helical pitch and a shorter reflective wavelength (blue shift).
The driving voltage, however, increases in direct ratio with the increased concentration of the chiral agent, resulting in various driving voltage among the cholesteric liquid crystal. FIG. 1 is a graph plotting the reflectance of RGB cholesteric liquid crystal materials against driving voltage of RGB cholesteric liquid crystal materials. As shown in FIG. 1, the driving voltages of RGB cholesteric liquid crystal materials VR, VG and VB are distinctively different (driving voltage of planar state represented by solid line, and driving voltage of focal conic state). Therefore, in a full-color display, there is necessary to provide three different driving voltages for driving red cholesteric liquid crystal material, green cholesteric liquid crystal material, and blue cholesteric liquid crystal material simultaneously.
There is, therefore, still a need for cholesteric liquid crystal materials with reduced driving voltage. Furthermore, the driving circuit design would be greatly simplified if cholesteric liquid crystal materials with different reflective wavelength are operated by a single driving voltage.